1. Field of the Invention
The invention relates to a method of regulating the membrane moisture of a polymer electrolyte fuel cell, and to a polymer electrolyte fuel cell comprising a means for regulating the membrane moisture. The fuel cells contain solid polymer membranes as electrolyte and preferably use hydrogen as burnable gas and air or oxygen under low pressure as oxidizing agent.
2. Description of the Related Art
Polymer electrolyte membrane fuel cells, as they are commonly employed for producing electric current, contain an anode, a cathode and an ion exchange membrane disposed therebetween. A plurality of fuel cells constitutes a fuel cell stack, with the individual fuel cells being separated from each other by bipolar plates acting as current collectors. For generating electricity, a burnable gas, e.g. hydrogen, is introduced into the anode region, and an oxidizing agent, e.g. air or oxygen, is introduced into the cathode region. Anode and cathode, in the regions in contact with the polymer electrolyte membrane, each contain a catalyst layer. In the anode catalyst layer, the fuel is oxidized thereby forming cations and free electrons, and in the cathode catalyst layer, the oxidizing agent is reduced by taking up electrons. The cations migrate through the ion exchange membrane to the cathode and react with the reduced oxidizing agent, thereby forming water when hydrogen is used as burnable gas and oxygen is used as oxidizing agent. In the reaction of burnable gas and oxidizing agent, there are released considerable amounts of heat that must be dissipated by cooling. Cooling so far has been achieved by cooling channels in the bipolar plates through which deionized water is flown.
With this kind of cooling, tremendous material problems result since there are typically about 50 to 300 bipolar plates connected in series, with the cooling water thus electrically joining together different potentials. The result thereof are material decompositions. In accordance therewith, solely graphite or gold-plated metal are feasible as material for the bipolar plates.
Furthermore, it is necessary to keep the polymer membrane moist, since the conductivity value of the membrane is greatly dependent on its water content. To prevent drying up of the membrane, there was thus required a complex system for humidifying the reaction gases.
It is the object of the invention to make available a polymer electrolyte fuel cell and a polymer electrolyte fuel cell stack, respectively, in which the polymer electrolyte membrane of a fuel cell has the optimum moisture content at all times during operation.
An additional object of the invention consists in making available a method which renders possible to keep the polymer electrolyte membrane of a polymer electrolyte fuel cell at an optimum moisture content during operation of the fuel cell.
The object is met by the method of regulating the membrane moisture of a polymer electrolyte fuel cell according to claim 1, the polymer electrolyte fuel cell according to claim 7 and the fuel cell stack of a plurality of polymer electrolyte fuel cells according to claim 12.
Preferred developments of the invention are indicated in the dependent claims.
Polymer electrolyte membranes require a high water content to ensure optimum conductivity for H+ ions. The water content must be maintained as a rule by supply of water, as otherwise the burnable gas flows and oxidizing agent gas flows flowing through the cell dry up the membrane. However, to counteract possible drying up by the addition of an excess of water, is not sensible since water in too large quantities results in flooding of the electrodes, i.e. the pores of the electrodes are clogged. Simple ascertaining and regulating the particular amount of water required has not been possible so far.